1. Field of the Invention
This invention relates to compositions and methods of using blends of nitrosophenol and C-nitrosoaniline compounds, optionally in combination with air or oxygen, to prevent the premature polymerization of ethylenically unsaturated monomers.
2. Description of the Related Art
Many ethylenically unsaturated monomers undesirably polymerize at various stages of their manufacture, processing, handling, storage and use. It is well known that ethylenically unsaturated monomers such as styrene or alpha-methylstyrene have a strong tendency to polymerize when subjected to elevated temperatures. Manufacturing processes for such monomers typically include distillations at elevated temperatures. A particularly troublesome problem is equipment fouling caused by polymerization during the manufacture of such monomers. Polymerization, such as thermal polymerization, results in the loss of the monomer and a loss in production efficiency owing to the deposition of polymer in or on the manufacturing equipment, the deposits of which must be removed from time to time. Additionally, the formation of soluble polymer leads to loss of monomer, i.e., a lower yield, and an increase in the viscosity of any tars that may be produced. The processing of the tars then requires higher temperature and work (energy cost) to remove residual monomer.
A wide variety of compounds have been proposed and used for inhibiting uncontrolled and undesired polymerization of ethylenically unsaturated monomers. These include elemental sulfur and many classes of organic chemicals. These materials have met with varying degrees of success in industrial use. Included among these organic materials are nitrosophenol and nitrosoaniline compounds, see, for example, F. Lartigue-Peyrou in Ind. Chem. Libr., 8 (Roots of Organic Development), 1996, 489–505; Prog. Polym. Sci., F. Tudos, T. Foldes-Berezsnich, 14, 1989, 717–761; Hung. 150,550; U.S. Pat. No. 4,654,451; U.S. Pat. No. 4,210,493; Eur. Polym. J., F. Tudos et. al., 18(4), 1982, 295–9; 19(7), 1983, 593–5; 8(11), 1972, 1281–9; 30(12), 1994, 1457–9; 19(3), 1983, 225–9; 19(2), 1983,153–18(6), 1982, 487–91, the contents of each of which are incorporated by reference herein
There remains a need, however, for an inhibitor that not only provides highly effective inhibition of polymerization during normal operation of a continuous manufacturing or purification process, but also provides satisfactory protection in the event of a loss of continuous inhibitor feed. While many inhibitors are known to provide sufficient protection in one of these scenarios, they have not been fully satisfactory under both normal and upset operating conditions.
Accordingly, a substantial need continues in the art for improved compositions for inhibiting the polymerization of such monomers during their production and during the distillation process for purifying or separating them from impurities, as well as during transport and storage.